A number of applications require temperature measurements in a number of different spatial locations at cryogenic temperatures (below 77K). These include superconductive cavity characterization for particle accelerators, superconducting power lines, NMR instrumentation and many more. Electronic temperature sensors (typically resistors or diodes of various types, less often thermocouples) while offering proven solution, have a number of deficiencies originating from poor multiplexing capability of such sensors as well as sensitivity to the magnetic fields.
Various types of fiber optics sensors are known to those skilled in the art, which offer high levels of multiplexing. These include Fiber-Bragg Grating (FBG) sensors, Fabry-Perot Interferometer, FPI, (extrinsic or intrinsic), Rayleigh scattering-based sensors, Raman scattering-based sensors and Brillouin scattering-based sensors (in scattering-based sensing systems typically sections of the fiber serve as sensors). Also incorporated here as a reference are fluorescence lifetime-based sensors. Fiber optic sensors are currently being used for a number of applications where high level multiplexing and/or Electromagnetic Interference (EMI) immunity are required. However, to date highly multiplexible fiber optic temperature sensing solutions were unable to meet the required levels of resolution and accuracy at cryogenic temperatures. Fluorescence lifetime-based fiber optic temperature sensors are the exception; however, such sensors are poorly multiplexible.
For example, FBG, FPI and Rayleigh scattering-based sensors are utilizing the combination of thermal expansion (characterized by coefficient of thermal expansion, CTE) and thermo-optical effect (characterized by thermo-optic coefficient) as mechanisms of transduction of temperature signal into optical signal.
At room temperatures and above the thermal expansion and thermo-optic coefficients of silica are sufficiently high to allow utilization of optical fiber itself or optical fiber with written FBG. However, thermal expansion coefficient [Johnson V J. Properties of materials at low temperature. Pergamon Press; 1961] and thermo-optic coefficient [Bradley J. Frey et al. Cryogenic temperature-dependent refractive index measurements of N-BK7, BaLKN3, SF15, and E-SF03; Proc. SPIE, Vol. 6692, 669205 (2007)] of silica decrease dramatically at cryogenic temperatures (see FIG. 1) preventing achievability of acceptable levels of accuracies and resolutions of fiber optic temperature sensors.
To address this problem, U.S. Pat. No. 6,072,922 teaches of using the coating with a thermal expansion coefficient that is larger than the thermal expansion coefficient of the optical fiber for increasing the sensitivity of the sensor to changes in temperature at the location. Both FBG and long period fiber grating are taught and aluminum (“Al”) (which can be integrated with the fiber, for example, through sputter depositing) or polymethyl methacrylate (“PMMA”) (which can be integrated as a coating, for example, by polymerization) are disclosed as possible realizations of coating materials. As it is illustrated in from FIG. 2, these materials indeed have significantly higher CTE than that of silica. While such a solution indeed showed improved levels of temperature resolution at cryogenic temperatures, it possessed two major deficiencies: 1) CTE of these materials is still significantly diminishes as temperature approaches ˜20K, 2) Significantly different CTEs of silica core of the fiber and the coating with a thermal expansion coefficient that is larger than the thermal expansion coefficient of the optical fiber are causing significant stress on the interface between the fiber and the coating. These stresses are often causing delamination of the coating and/or breakage of the fiber. Sham-Tsong Shiue et al. [Effect of coating thickness on thermal stresses in tungsten-coated optical fibers, J. Appl. Phys., Vol. 87, No. 8, 3760 (2000)] showed that for metal coating (tungsten coating was considered in detail) maximal metal coating thickness exists (˜100-200 nm) after which delamination and/or fiber breakage is inevitable upon cooling the coated fiber. R. Rajini-Kumar et al. [Performance evaluation of metal-coated fiber Bragg grating sensors for sensing cryogenic temperature Cryogenics 48 (2008) 142-147] recently studied aluminum (Al), copper (Cu), lead (Pb) and indium (In) coated FBG sensors and demonstrated that indium and lead coating enlarge the operational range of fiber optic sensors down to ˜15K (see FIG. 3). Still reliable, multiplexible fiber optic sensors for liquid He temperatures (2K to 5K) are missing.